x is by default interpreted as BracketingBar[x].
Used in mathematics to indicate absolute value (Abs), determinant (Det), and other notions of evaluating size or magnitude.
Not the same as \[VerticalBar].
Drawn in monospaced fonts with a small left-pointing tee to indicate direction.
See The Mathematica Book: Section 3.10.1 and Section 3.10.4.
See also: \[LeftDoubleBracketingBar] , \[LeftTee] .